Probe cards employing probes having retaining portions for potting in a potting region

ABSTRACT

Method and apparatus using a retention arrangement for probes used for electrical testing of a device under test (DUT). The apparatus has a number of probes each of which has a connect end for applying a test signal, a retaining portion, at least one arm portion and a contact tip for making an electrical contact with the DUT. A retention arrangement has a tip holder for holding each of the probes by its contacting tip and a plate with openings for holding each of the probes below the retaining portion. The retaining portion of each of the probes is potted in a potting region defined above the plate with the aid of a potting agent. The apparatus can be used with space transformers, a variety of probes of different geometries and scrub motion characteristics and is well-suited for use in probe card apparatus under tight pitch and small tolerance requirements.

RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.11/302,650 filed on Dec. 14, 2005 and is herein incorporated in itsentirety.

FIELD OF THE INVENTION

This invention relates generally to apparatus and method for retainingprobes in an electrical testing arrangement such as a probe card, andmore specifically to potting of such probes in a retention arrangementbelonging to the probe card to ensure superior mechanical and electricalperformance.

BACKGROUND ART

The testing of semiconductor wafers and other types of integratedcircuits (ICs), collectively known as devices under test (DUTs), needsto keep pace with technological advances. Each IC has to be individuallytested, typically before dicing, in order to ensure that it isfunctioning properly. The demand for testing products is driven byconsiderations of new chip designs and higher volumes.

In particular, chips are getting smaller and they have more tightlyspaced contact pads. The pads are no longer located about the circuitperimeter, but in some designs may be found within the area occupied bythe circuit itself. As a result, the spacing or pitch of probe arraysrequired to establish electrical contact with the pads or bumps isdecreasing. In addition, the requirements on planarity are increasing.

Some of the problems associated with small pitch of contact pads andtheir arrangement in a dense two-dimensional array are addressed in theprior art. For example, U.S. Pat. No. 6,881,974 to Wood et al. teachesto arrange probes in a probe card that has a substrate with a pluralityof blind holes on a front face. The holes are filled with a metal toestablish electrical contact for testing purposes and have closedbottoms spaced from a back of the substrate. Another approach is taughtin U.S. Pat. No. 6,586,955 and U.S. Published Application No.2002/0000815 both to Fjelstad et al. where probe cards include a layerof dielectric material provided with a plurality of cavities on asubstrate. The dielectric material separates the fusible conductivematerial that is used to form the probe contacts. Both approachesprovide ways to arrange probes that can address pads in a dense array.

In an approach that employs a housing for holding the individual probes,U.S. Pat. No. 6,566,898 to Theissen et al. teach an improved verticalpin probing device that has upper and lower spacers made of Invar. Thespacers have a thin sheet of silicon nitride ceramic material held in awindow in the spacer by an adhesive. The sheets of silicon nitride havelaser-drilled matching patterns of holes supporting the probe pins andinsulating them from the housing. The probes held in the holes can bearranged to address pads in a dense array.

Unfortunately, merely providing a geometry or method of holding probesthat can address a dense array of pads is not sufficient. It is alsoimportant to ensure that the probes have suitable mechanical resilienceand compliance properties. One way of addressing these mechanical issuesis discussed, for example, in U.S. Published Application No.2002/0117330 to Eldridge et al. This reference teaches structures thathave improved resilience or compliance because the wire used for contactis overcoated with at least one layer of a material chosen for itsstructural resiliency or compliance characteristics. The probes havespringy shapes and are attached to a substrate in, e.g., a probe card.

Although the prior art solutions individually address some of theproblems, there is no apparatus or method that combines the requisitecharacteristics in a single probe card or testing apparatus.Specifically, what is needed is an electrical testing apparatus that canaddress densely packed pads or bumps with probes that are held securelywhile offering requisite mechanical properties such as resilience,compliance and reliable scrub motion to remove oxide from the pads orbumps.

OBJECTS AND ADVANTAGES

In view of the above prior art limitations, it is an object of theinvention to provide for a method and apparatus for electrical testingof devices under test (DUTs) that have densely spaced contact pads orbumps. The object is to ensure that the probes used in such apparatusare appropriately held and designed to ensure advantageous mechanicalproperties, including resilience, compliance and scrub motion.

It is another object of the invention to ensure that the apparatus iseasy to assemble and disassemble despite the small pitch of the probes.

These and other objects and advantages of the invention will becomeapparent from the ensuing description.

SUMMARY OF THE INVENTION

The objects and advantages of the invention are secured by a method andan apparatus for electrical testing of a device under test (DUT). Theapparatus has a number of probes each of which has a connect end forapplying a test signal, a retaining portion, at least one arm portionand a contact tip for making an electrical contact with the DUT.Further, the apparatus has a retention arrangement for holding each ofthe probes. Specifically, the retention arrangement has a tip holder forholding each of the probes by its contacting tip and a plate withopenings for holding each of the probes below the retaining portion. Theretaining portion of each of the probes is potted in a potting regiondefined above the plate with the aid of a potting agent. The pottingagent can be any suitable potting material such as an epoxy selected forappropriate viscosity and curing properties. In some embodiments theapparatus further comprises a lateral enclosure or, more generally alateral barrier for enclosing the potting region to help contain theepoxy prior to curing.

In a preferred embodiment the openings in the plate are laser-machinedopenings. The laser machining process allows one to achieve very precisedimensional tolerances and opening profiles.

In some applications the apparatus is used in conjunction with a spacetransformer. The space transformer has closely spaced or high pitchcontacts for contacting each of the probes at the connect end. Theconnection can be permanent or, preferably temporary. Suitable temporaryconnections can be achieved by soldering. The use of the spacetransformer is particularly advantageous when the apparatus is employedin a probe card.

The apparatus admits the use of many types of probes. Preferably,however, the probes are non-linear. For example, each probe has at leasttwo arm portions joined by a knee. In one specific embodiment, theprobes have at least two arm portions with a base arm portion extendingaway from a center axis of the probe and a reverse arm portion extendingtoward the center axis. The knee joins the base arm portion with thereverse arm portion. The contacting tip is located on the reverse armportion distal from the knee and the contacting tip has a non-zerooffset relative to the center axis. Alternatively, the probes can haveat least one non-linear arm portion that extends from the center axissuch that the contacting tip again exhibits a non-zero offset relativeto the center axis. In any of the embodiments of the apparatus, theprobes can be of the type endowed with a protrusion on the contactingtip for ensuring high quality electrical contact.

The invention further extends to a method for electrical testing of aDUT with a number of probes each having a connect end for applying atest signal, at least one arm portion and a contacting tip for making anelectrical contact with the DUT. Each of the probes is provided with aretaining portion and a retention arrangement is provided for holdingeach of the probes by its contacting tip and also below its retainingportion. According to the method a potting region is created for pottingthe retaining portion of each probe in a potting agent. The pottingagent is admitted into the potting region once the probes are properlyheld to pot the retaining portions of the probes.

Preferably, the retention arrangement has a plate with holes for holdingeach of the probes below its retaining portion and the holes are made bylaser machining. Additionally, the retention arrangement has a tipholder for holding each of the probes by its contacting tip. The tipholder can be removed after potting or left in place for testing.

According to the method, a space transformer can be provided forcontacting the probes at their connect ends and applying the testsignals thereto.

A detailed description of the preferred embodiments of the invention ispresented below in reference to the appended drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a diagram illustrating an apparatus of the invention employedwith a space transformer.

FIG. 2 is an isometric view of two probes that can be used in accordancewith the invention.

FIG. 3 is an isometric view of another probe that can be used inaccordance with the invention.

FIG. 4 is a three-dimensional view of other probes with non-linearportions.

FIG. 5 is a three dimensional view of still other probes with non-linearportions.

FIG. 6 is a three-dimensional view illustrating an array of probes heldin a retention arrangement.

FIG. 7 is a three-dimensional view illustrating the operation of ascrubbing protrusion on a contacting tip of a probe from the array ofFIG. 6.

FIG. 8 is a three-dimensional view illustrating the array of probes andretention arrangement of FIG. 6 attached to a space transformer inaccordance with the invention.

FIG. 9A is a plan cross sectional view illustrating a method ofattaching probes held in a retention arrangement of FIG. 9 to a spacetransformer.

FIG. 9B is a plan cross sectional view illustrating the operation ofprobes in the retention arrangement of FIG. 9A when the holder is notremoved.

DETAILED DESCRIPTION

The present invention will be best understood by first reviewing anapparatus 10 of the invention as shown in the diagram of FIG. 1.Apparatus 10 can be employed in a probe card or other electrical testingequipment for testing a device under test (DUT) 12. Frequently, DUT 12is an integrated circuit on a wafer that requires testing prior todicing. Alternatively, DUT 12 is an electronic device or circuit that isalready mounted. The functionality of DUT 12 is verified by applyingtest signals to a number of its bumps or pads 14.

Apparatus 10 has a number of probes 16 arranged in an array and designedfor establishing electrical contact with pads or bumps 14. Typically,the number of probes 16 is large and their spacing or pitch is verysmall, e.g., on the order of several microns. For clarity, only threeprobes 16A, 16B, 16C are illustrated. The construction of all probes 16is analogous and will be explained by referring explicitly to probe 16A.

Probe 16A has a connect end 18A for applying the test signal, aretaining portion 20A and two arm portions 22A, 24A. Arm portion 24Aterminates in a contacting tip 26A for making electrical contact with acorresponding bump 14A of DUT 12.

Apparatus 10 has a retention arrangement 28 for holding each of probes16 below its retaining portion 20 and just above arm portions 22, 24.More precisely, retention arrangement 28 has a plate 30 with openings32. Preferably, plate 30 is a ceramic plate. Openings 32 are providedfor receiving and guiding retaining portions 20 of probes 16. To ensureaccurate placement of probes 16 in retention arrangement 28 openings 32are precision machined. Preferably, openings 32 are laser-machinedopenings.

Further, retention arrangement 28 has a holder 34 for holding probes 16by their contacting tips 26. Holder 34 can be made of various materials,but in the present case it is made of polyimide. A series of holes 36 inholder 34 is designed to retain contacting tips 26. Holes 36 preferablyhave a suitable structure or cross-section to ensure that, onceinserted, contacting tips 26 stay in holes 36 unless intentionallyremoved. A frame 38 keeps holder 34 in the appropriate position withrespect to plate 30 to ensure that probes 16 are all well-aligned andtheir contacting tips 26 maintain a high level of planarity.

Retaining portions 20 of probes 16 are potted with a potting agent 43 ina potting region 40 defined above plate 30. A lateral enclosure 42, herein the form of a lateral barrier mounted on top of and about theperimeter of plate 30 defines potting region 40. A person skilled in theart will recognize, that lateral enclosure 42 is especially useful forcontaining any low-viscosity potting agent 43 prior to curing or when asignificant thickness of potting agent 43 needs to be employed. Asuitable potting agent 43 is an epoxy that exhibits proper wetting withrespect to plate 30 and retaining portions 20 of probes 16 and hardensupon contact with atmospheric oxygen. Potting agent 43 may be pouredinto potting region 40 from above once probes 16 are properly alignedboth horizontally and vertically. As potting agent 42 cures and hardens,probes 16 are retained in their proper positions.

Apparatus 10 also has a space transformer 44 with probe contacts 46 onits bottom surface 48 for contacting probes 16 at their connect ends 18.Specifically, space transformer 44 is employed for establishingelectrical connections between test signal leads 50 from a testingdevice 52, e.g., a testing circuit on a printed circuit board, andprobes 16. In contrast to signal leads 50, probe contacts 46 on bottomsurface 48 are densely spaced and can be directly connected to probes16. For example, in the case of probe 16A, its connect end 18A isassigned to establish electrical connection with contact 46A. Inpractice this is preferably done by providing a soldering ball oncontact 46A and soldering connect end 18A thereto. Other alternatives,such as a side friction connector between connect end 18A and contact46A can also be used to establish electrical connection.

Space transformer 44 allows the user to convert relatively sparselyspaced leads 50 to an array of much more densely spaced or high pitchprobe contacts 46 for accessing very densely spaced probes 16. Probes16, in turn, require tight spacing in order to access very denselypacked and small pads or bumps 14 of DUT 12. Various types of spacetransformers and routing strategies are known to those skilled in theart. Any of those can be applied in apparatus 10. In addition, testingdevice 52 can generate test signals directly, receive externalinstructions for generating test signals or simply receive some or allof the test signals and assign them to signal leads 50.

During operation, probes 16 of apparatus 10 are placed in physicalcontact with bumps 14 to establish electrical contact. Electricalcontact is not only due to physical contact, but also due to a scrubbingmotion executed by contacting tips 26 of probes 16 while engaging withbumps 14. The scrubbing motion removes surface oxidation from bumps 14and thus ensures a low-resistance electrical contact so that the testsignals are efficiently delivered to bumps 14.

Apparatus 10 can employ probes of various types and geometries,including probes with two or more arm portions. FIG. 2 is an isometricview of two exemplary probes 60A, 60B that can be used in apparatus 10.Each one of probes 60 has a retention portion 62A, 62B and two armportions 64A, 66A and 64B, 66B, respectively. More precisely, armportions 64A, 64B are base arm portions extending away from center axesAA, AB of probes 60A, 60B, and arm portions 66A, 66B are reverse armportions extending back toward center axes AA, AB. Base and reverse armportions 64A, 64B and 66A, 66B inflect at corresponding joints or knees68A, 68B. This geometry lends probes 60A, 60B a measure of mechanicalflexibility that allows contacting tips 70A, 70B of probes 60A, 60B toperform effective scrubbing movements when brought in contact with padsor bumps 14 of DUT 12.

Probes 60 have round cross-sections and are spaced at a pitch Δ. In apreferred embodiment, contacting tips 70A, 70B are located on reversearm portions 66A, 66B distal from knees 68A, 68B with a non-zero offsetδ relative to center axes AA, AB, respectively. Non-zero offset δfurther improves the scrubbing behavior of probes 60.

Apparatus 10 can use other probes that have non-circular cross-sections.FIG. 3 illustrates in an isometric view a probe 80 that has a generallyrectangular and varying cross-section. Probe 80 has a retention portion82, a base arm portion 84, a reverse arm portion 86, a knee 88 and acontacting tip 90. Once again, contacting tip 90 has a non-zero offset δrelative to a center axis AA of probe 80 to achieve improved scrubbingmotion.

In still other embodiments, apparatus 10 uses probes that have one ormore non-linear arm portions. FIG. 4 illustrates an array of probes 100each having a retention portion for being potted in retentionarrangement 28 and a contact end 104. Each probe 100 has a non-lineararm portion 106 with a knee 108. Arm portion 106 terminates in acontacting tip 110. Note that contact end 104 of each probe 100 isdesigned for making side friction connections rather than a solderedconnection. In other words, contact end 104 is designed to be placedinto a metallized hole of a ceramic plate located above retentionarrangement 28 and contact is established by sliding the ceramic platesideways to ensure mechanical contact with contact end 104. In this casespace transformer 44 establishes electrical connections with contact end104 via a soldered connection to the ceramic plate. Alternatively, spacetransformer 44 may itself be provided with metallized holes forreceiving contact end 104 of each probe 100. Also note, that in thisembodiment there is no offset between contacting tip 110 and the centeraxis of probe 100.

FIG. 5. shows an array of probes 120 each having a retention portion 122terminated in a contact end 124. A stop 126 is provided at the lower endof retention portion 122 to help align probes 120 in a retentionarrangement and aid in maintaining their planarity. The presence of stop126 also aids in keeping the potting agent in the potting region duringassembly.

Each probe 120 has a non-linear arm portion 128 with a joint or knee130. Non-linear arm portion 128 has a varying degree of curvature alongits length and terminates in a contacting tip 132. Tip 132 is offsetfrom the center axis of probe 120 in order to provide for improvedscrubbing behavior.

FIG. 6 is a three dimensional view illustrating an array of probes 140each having a non-linear arm portion 142 and a contacting tip 144.Probes 140 are mounted in a retention arrangement 146 and extend outfrom plate 148 through openings 150. The holder for holding probes 140at contacting tips 144 has been removed in this embodiment. Arrangement146 is fully assembled and probes 140 are potted in the potting agent inthe potting region (not visible) of arrangement 146.

FIG. 7 illustrates contacting tip 144 of a particular probe 140 in moredetail. The bottom surface of tip 144, which is the surface that comesin contact with a pad or bump 154, is provided with a scrubbingprotrusion 152. Protrusion 152 is a raised, rectangular portion of thebottom surface of tip 144. Although other geometries are possible, thistype of protrusion 152 is preferred. During operation, as a contactforce is applied, tip 144 comes in contact with bump 154 and executes alateral scrubbing motion, as indicated by arrow S. The scrubbing motionhelps to remove oxide from bump 154 and establish a good electricalcontact.

Terminating the tips with scrubbing protrusions in any apparatus of theinvention is preferred, since it improves the scrubbing behavior of theprobes. It should also be noted, that due to the improved hold of theprobes achieved by potting them in the potting enclosure, as well as anyaxial offset of their tips, all of these measures cooperate to produce avery effective scrubbing movement.

FIG. 8 is a three-dimensional view of the array of probes 140 inretention arrangement 146 attached to a space transformer 156. In thisembodiment connections to space transformer 156 contacts are solderedconnections 158. Once completed, connections 158 can be capped with anepoxy or adhesive for protection.

FIG. 9A is a plan cross sectional view illustrating a set of probes 160in a retention arrangement 162 that employs plate 164 and holder 166.Plate 164 has openings 168 for holding probes 160 below their retainingportions 170. Holder 166 has openings 172 for keeping contacting tips174 of probes 160. A potting region 176 is defined above plate 164.Potting region 176 does not include any lateral barriers for containinga potting agent 178. In this embodiment, agent 178 is a sufficientlyviscous epoxy to not require containment prior to curing. In addition,epoxy 178 may be applied and cured in layers to build up to the requiredthickness without the need for lateral containment.

In the view shown by FIG. 9 probes 160 are already potted in pottingagent 178 and are being attached to a space transformer 182. Toestablish the electrical connection, connect ends 184 or probes 160 arebrought in contact with and soldered to pads 186 of transformer 182 byre-flowing solder balls 188. Preferably, an underfill 180 (see FIG. 9B),typically of an epoxy or other stable dielectric is provided in order tostrengthen the mechanical connection between transformer 182 andretention arrangement 162.

FIG. 9B is a plan cross sectional view illustrating the use of probes160 mounted in retention arrangement 162 with holder 166 left in placeover contacting tips 174 rather than removed. Due to the presence ofholder 166 the motions and scrubs, as indicated by arrow S, of probes160 are mechanically coupled. Thus, as contacting tips 174 engage withcontact pads or bumps 200 of a DUT 202 they will tend to execute moreconcerted scrub motion. Of course, a person skilled in the art willrecognize that the decision to remove or keep holder 166 can be made ona case by case basis and depending on planarity, scrub length andcontact force requirements when dealing with any particular DUT 202.

Any of the above-described embodiments can be implemented in afull-fledged testing system or probe card. The retention arrangement ofthe invention provides excellent mechanical characteristics to theprobes it holds. In particular, even in very dense arrays that addressdensely packed probe pads or bumps the probes are held securely whileoffering requisite mechanical properties such as resilience, complianceand reliable scrub motion to remove oxide from the contact pads orbumps.

Many other embodiments of the apparatus and method are possible.Therefore, the scope of the invention should be judged by the appendedclaims and their legal equivalents.

1. An apparatus for electrical testing, comprising: a) a plurality ofprobes, each of said probes having: I. a connect end for applying a testsignal; II. a retaining portion; III. at least one arm portion; and IV.a contacting tip for making an electrical contact with a device undertest; b) a retention arrangement for holding each of said probes, saidretention arrangement having: I. a tip holder for holding each of saidplurality of probes by said contacting tip; II a plate having openingsfor holding each of said probes below said retaining portion; and III. apotting region above said plate for potting said retaining portion in apotting agent.
 2. The apparatus of claim 1, further comprising a lateralbarrier for enclosing said potting region.
 3. The apparatus of claim 1,wherein said potting agent comprises an epoxy.
 4. The apparatus of claim1, wherein said openings are laser-machined openings.
 5. The apparatusof claim 1, further comprising a space transformer having contacts forcontacting said probes at said connect end.
 6. The apparatus of claim 1,wherein said probes each comprise at least two arm portions.
 7. Theapparatus of claim 6, wherein said at least two arm portions comprise abase arm portion extending away from a center axis of said probe, areverse arm portion extending toward said center axis, and said probefurther comprises a knee joining said base arm portion with said reversearm portion.
 8. The apparatus of claim 7, wherein said contacting tip islocated on said reverse arm portion distal said knee and said contactingtip has a non-zero offset relative to said center axis.
 9. The apparatusof claim 1, wherein said at least one arm portion comprises a non-lineararm portion.
 10. The apparatus of claim 9, wherein said non-linear armportion extends away from a center axis of said probe such that saidcontacting tip has a non-zero offset relative to said center axis. 11.The apparatus of claim 1, wherein said contacting tip has a protrusionfor making said electrical contact.
 12. The apparatus of claim 1,employed in a probe card.
 13. A method for electrical testing of adevice under test using a plurality of probes each having a connect endfor applying a test signal, at least one arm portion and a contactingtip for making an electrical contact with said device under test, saidmethod comprising: a) providing a retaining portion in each of saidprobes; b) providing a retention arrangement for holding each of saidprobes by said contacting tip and below said retaining portion; c)creating a potting region for potting said retaining portion in apotting agent; d) admitting a potting agent into said potting region forpotting said retaining portion of said probes.
 14. The method of claim13, wherein said retention arrangement is provided with a plate havingopenings for holding each of said probes below said retaining portion.15. The method of claim 14, further comprising laser machining saidopenings.
 16. The method of claim 13, wherein said retention arrangementis provided with a tip holder for holding each of said probes by saidcontacting tip.
 17. The method of claim 16, wherein said tip holder isremoved after potting.
 18. The method of claim 13, further comprisingproviding a space transformer for contacting said probes and applyingsaid test signal at said connect end.